The background of the invention will be set forth in two parts.
1. Field of the Invention
This invention relates to surface acoustic wave devices and more particularly to such device incorporating improved interdigital transducers.
2. Description of the Prior Art
Basically, a surface acoustic wave (SAW) device comprises a source of an electrical input signal, a smooth slab-like element or substrate of material capable of propagating such acoustic energy, and a load or utilization device. Additionally, electroacoustic transducers are attached to the substrate to convert the input signal to surface waves in the material, and vice versa.
In the most common designs, a piezoelectric material such as lithium niobate or quartz, for example, is used as the surface wave medium, and the transducers employed are the interdigital array type. This type of array consists of a series of conductive electrodes that form a comb-like pattern which is disposed on the piezoelectric substrate surface. The interdigital transducer is a two-terminal device having two separate components of metal strips resembling interleaved fingers. In the case of an input transducer, the adjacent fingers have predetermined potentials and polarities causing a strain field to be set up between the electrodes in the surface of the substrate. The surface acoustic wave energy created by the strain field propagates along the substrate surface and creates potentials in adjacent electrodes of an output transducer disposed in the path and intercepting such propagating energy.
The gap between adjacent electrodes can be viewed as an isolated source of acoustic signal (or tap when viewed as a receiver element). The response of this type of transducer is thus the sum of the sources in the array. In order to realize a prescribed response, it is necessary to control the relative amplitudes of the respective sources.
The standard prior art technique used to provide this desired control is to vary the electrode overlap dimensions proportional to the required weighting. This is commonly denoted as "apodization", which is described in many articles on the subject, one of which is "Acoustic Surface Wave Filters" by R. H. Tancrell and M. G. Holland, in the Proceedings of the IEEE, Vol. 59, pages 393-409, March 1971.
Although providing the desired primary result, this technique has certain undesirable second order effects where there is a relatively small electrode overlap (less than 25%, for example). There are two limitations involved: first, is acoustic diffraction, or beam spreading, which produces errors in the amplitude and phase of the source radiation. These errors become significant when the overlap dimension is small. The second problem is that acoustic radiation emanates from the region beyond the ends of the electrodes, in addition to the overlap region. This component produces a significant error in the effective tap weight when the overlap dimension is small. In a practical prior art SAW filter, for example, these errors limit achievable out-of-band rejection to roughly -30 dB and produce similar errors in other types of SAW devices.
There are still other significant problems to contend with in prior art SAW devices using apodization. For example, it is very difficult to acquire and maintain the close tolerances required in the fabrication of the transducers on the substrate surface, and in the prior technology, only one of the two transducers in a filter pair could be accurately weighted. Further, once the mask used for fabricating the apodized transducer has been completed, the impulse response cannot be modified.
As can be seen from the above, amplitude weighting is a general requirement for all surface acoustic wave transducers. That is, it is always necessary that the impulse response of the surface wave transducer has some prescribed amplitude variations. In contrast to the prior art techniques, the invention as will herein be described utilizes resistive weighted transducers which have the advantages of being more accurate and easier to fabricate than through the use of prior techniques. Furthermore, the present implementation is compatible with the use of two resistive weighted transducers, combining to form a single surface acoustic wave filter, as compared to prior technology where only one transducer in a filter pair could be accurately weighted. The present invention may also yield twice the out-of-band filter rejection that was previously obtainable. An additional advantage of the resistive weighted transducer technique lies in the relative ease with which the amplitude of the measured impulse response can be trimmed and modified in comparison with prior techniques.
Another prior art technique which should perhaps be mentioned utilizes the selected positioning of the tapping electrode at prescribed angles to the incident surface acoustic wave beam. As this latter technique offers far less flexibility than either apodization or the resistive weighted technique, and since the latter is also far more sensitive to fabrication errors, it has not received wide usage.